Composite hydrogen transport membranes are used to separate hydrogen from a hydrogen containing feed for a variety of industrial processes. Such membranes can consist of a hydrogen transport material supported by a porous support. It is believed that such membranes function by diffusing the hydrogen to a surface of the membrane then disassociating the hydrogen molecules to hydrogen ions and electrons. The hydrogen ions and electrons are transported through the membrane and at an opposite surface of the membrane, the hydrogen molecules recombine and gain the electrons. Hydrogen then diffuses away from such opposite surface of the membrane.
Such hydrogen transport membranes are fabricated from palladium or alloys of palladium containing silver and function at high temperatures under the driving force of pressure. The flux of the hydrogen developed through the membrane is in part dependent upon the diffusion resistance of the membrane, the pressure applied and the temperature to which the membrane is subjected.
In order to reduce the diffusion resistance through the membrane, the membrane should be made as thin as possible so as to provide the shortest path of diffusion for the hydrogen through the membrane. Since, as stated above, such hydrogen transport materials are subjected to a driving force of pressure, thin layers of hydrogen transport material must be supported on a porous support, typically a ceramic.
In the prior art, hydrogen transport materials have been supported within the pores of the ceramic, solely on the surface of the ceramic or a combination of both methods of support. For example, in U.S. Pat. No. 6,066,592, both methods of support are used. In fabricating the membrane in accordance with the teachings of this patent, one surface of a porous support is immersed in a solution containing a sensitized metal so that pressure can be applied to one surface that is higher than the pressure of the opposite surface of the support. The pressure forces the sensitized metal to be deposited not only on the surface of the support but also on inner surfaces of the pores. Thereafter, an electroless plating technique is carried out utilizing a plating solution containing a reducing agent to deposit palladium onto the porous support so that the metal fills the pores and closes them.
In U.S. Pat. No. 6,899,744, a hydrogen transport membrane is formed in which a saturated solution of palladium chloride is placed on one side of a porous support and a water soluble organic reducing agent is placed on the opposite side of the support. The reagents diffuse through the porous support and react to deposit palladium in the pores.
A disadvantage of a membrane formed in a manner that involves filling the pores of a support with the hydrogen transport material is that the distance for the hydrogen to diffuse is greater than that possibly attainable with a thin metallic coating of material on the support. Additionally, the diffusion resistance of the hydrogen through the support has invariably increased due to the plugging of the pores.
U.S. Pat. No. 6,761,929 discloses a method of producing a coating of the hydrogen transport material on a support in which the degree to which the material penetrates the pores is controlled by pressurizing the surface of the support that is located opposite to the surface that is to be plated by an electroless plating technique. In this manner, a hydrogen transport material is not able to penetrate the pores and a uniform thickness of material is able to develop at the surface of the support. A problem with this technique is that it presents a limitation in the ability to obtain very thin layers of hydrogen transport materials. The reason for this is that as hydrogen transport material bridges the pores, such material is also being deposited at locations of the supporting support situated between the pores.
As will be discussed, the present invention relates to a method of forming a hydrogen transport membrane in which porosity of the support is maintained in an open condition while very thin, defect-free hydrogen transport material layers are able to be produced at the surface of the support.